Ammonia fountain experiment – compared with HCl and N₂
ChemistryInorganic ChemistryAges 16–17Virtual lab
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Inorganic Chemistry
An inverted round-bottomed flask full of NH₃ gas has a tube dipping into a trough of water containing phenolphthalein. Squeeze the dropper to add a few drops of water: NH₃ is extremely soluble (≈ 700 volumes per volume of water), so the pressure in the flask drops sharply and water is forced up as a pink fountain. Switch to HCl gas (with litmus or methyl orange) or to poorly soluble N₂ to compare; a pressure gauge and a p(t) graph update continuously.
Lesson: Ammonia – solubility in water and the basic (alkaline) nature of aqueous ammonia
What it shows
Ammonia is one of the most soluble gases known: at 20 °C one volume of water dissolves roughly 700 volumes of NH₃. When a little water enters a flask of ammonia, most of the gas dissolves at once, the pressure inside falls far below atmospheric pressure, and the atmosphere pushes water up the tube as a fountain. The solution formed is alkaline because NH₃ + H₂O ⇌ NH₄⁺ + OH⁻, so the indicator changes color as the water enters. Hydrogen chloride gives a similar fountain with an acidic solution, while nitrogen gives none.
How to use
Keep ammonia and phenolphthalein, press Squeeze dropper and watch the pressure gauge, the p(t) graph and the color of the water entering the flask. Repeat with hydrogen chloride and each Indicator in the trough water, then with nitrogen, and compare the rows of the results table. Changing the Flask volume shows that only a tiny amount of water is needed to dissolve all the gas.
Parameters you can change
- Gas in the flask Ammonia NH₃, Hydrogen chloride HCl, Nitrogen N₂ (poorly soluble)
- Indicator in the trough water Phenolphthalein, Litmus, Methyl orange
- Flask volume 250–1000 mL
- Simulation speed 0.5–2 ×
Questions to explore
- Why do a few drops of water make water spray into a flask of NH₃, but not into a flask of N₂?
- With NH₃ in the flask, what color does the water turn with phenolphthalein and with litmus, and why?
- Which indicator shows the HCl fountain clearly, and why is phenolphthalein unsuitable for it?
